The purpose of this work is to collect complete family information on individuals born in Denmark between 1941 and 1968 with a cleft of the primary and/or secondary palate. This information will be used to test the polygenic model of inheritance with a sex-modified threshold as proposed by Carter. To date the information collection on isolated cleft palate families is essentially complete with over 500 pedigrees recorded for study. Data analysis on this population has not yet begun. Complete information has been obtained on 81 twins pairs of both zygosities in which at least one twin has a cleft. Seven of 13 MZ pairs with cleft lip t cleft palate (CLP) were concordant for a rate of 53%. For isolated cleft palate (CP), 3 MZ pairs had a concordance rate of 33%. The DZ concordance rates were very low for both CL(P) and CP emphasizing the important contribution of heredity to these entities. Finally, analysis of 1323 sibships, in which there was at least one individual with a cleft of the primary palate (CLP), showed that the general predictions of the Carter Model are supported. However, we did not find a deficiency of normal males in sibships with multiple affected as might be expected. A trend for a decreased M/F sex ratio in sibships with 3 or more affected was noted but the difference was not significant.